Current color silver halide papers utilize three color layers comprised of a red sensitive cyan layer, a green light sensitive magenta layer and blue light sensitive yellow layer. In order to increase performance of color silver halide paper in digital printers, there is a strong desire to have high gamma in all three color records. Due to the large grain size of the blue light sensitive yellow emulsion, the layer location of conventionally designed blue light sensitive yellow layers, and the high intensity reciprocity performance of conventionally sensitized blue light sensitive yellow emulsions, the gamma of the blue color record is usually considered to be a limiting factor. These effects are especially evident in areas of high dye density, that is in the shoulder and Dmax (area of maximum density) regions. For instance, in the case of the blue sensitive layer, a low shoulder would lead to black areas going blue and yellow colors desaturating. One common way to increase gamma is increase the silver coverage of the emulsion. However, increased silver causes a number of problems.
In particular, higher silver coverage leads to a reduction in the developability of the photographic element. This results in a lowering of neutral gamma for a given silver coverage at a given time of development, and an increase in the variability of gamma for a non-fully developed silver halide product.
A further effect of using high coverage of emulsion is a deleterious loss of color purity. This is caused by the tendency for the oxidized developer (Dox) to be formed in too high of a concentration to be used effectively by the color forming coupler. Thus, more of the Dox wanders from the color record in which it was formed into adjacent layers. Typically, color photographic elements contain Dox scavenging interlayers (otherwise known as anticolor-mixing layers) to prevent interlayer color contamination. In the case of large grained emulsions, the scavenging layer must be made thicker in order to react with excess Dox and prevent it from reaching another color forming layer. This also results in materials being wasted, since no dye is formed as a result of this process.
Further, there are practical limitations on how much gelatin and/or Dox scavenger can be added to a photographic element. For example, additional gelatin can not be added to some commercial color print materials without slowing down the coating operation of the element. Further, the addition of gelatin may also lead to prints sticking together after conventional processing because of dryer limitations in some processors. The addition of more Dox scavenger also has inherent difficulties. In particular, as the oil fraction of the interlayer is increased, relative to the fraction of gelatin, the layer tends to become less robust to physical abuse. One consequence is that the integrity of the layer can become comprised, and under certain stresses such as scratches occurring during processing, the photographic element will tend to break at the interlayer leaving only the bottom layer in this area.
A further problem can be the migration of the Dox scavenger (typically a hydroquinone, hydrazide or sulphonamidophenol) into the magenta imaging layer. In the case of the magenta coupler being a pyrazoloazole this leads to two concerns. The first is that many pyrazoloazoles couplers are somewhat less reactive toward oxidized developer than is the interlayer scavenger. This leads to less magenta dye formation in the regions of scavenger migration. A second impact is that some Dox scavengers (in particular, hydroquinones) destabilize the magenta dye when it is exposed to light.
In some instances, the very large grained emulsions produce a local concentration of Dox that is too high to be completely used by the appropriate coupler and/or scavenged by an anticolor mixing agent. For instance, in the case of large grained emulsions used in the blue layer, this can result in the formation of magenta colored spots in the photographic print from reaction of Dox formed in the blue layer with magenta coupler situated in the green layer.
Another approach to increasing blue gamma would be to increase the yellow coupler coverage in the blue light sensitive layer. This approach does not yield substantial gains in gamma due to the excess of coupler already coated in the layer. Also, substantial increases in coupler will increase levels of non-matrixed organics, which will damage the structural integrity of the gelatin matrix.
U.S. Pat. No. 4,040,829 describes a structure where a semi-diffusable coupler layer is coated on top of the topmost emulsion layer. However, in this invention the intent is to use the same or another non-diffusable image coupler in the adjacent enhancer layer. In a preferred embodiment, the non-diffusable coupler would be the same in both layers and would be used in the bottom-most layers of the element.
European Patent Application No. 0 062 202 describes a structure in which the emulsion layers are sandwiched between two coupler containing layers. The structure of the current invention differs in that it requires coating a coupler containing layer above a coupler and silver containing layer.
Japanese Kokai Patent Application No. Sho 531978!-65730 teaches using an additional 0.01-0.3 g/m.sup.2 of yellow coupler in the interlayer between the blue light sensitive layer and the green light sensitive layer. The inventive structure differs because the additional coupler is contained in an enhancer layer which is sandwiched between a coupler and silver containing layer on one side and an anticolor mixing layer on the other side. In addition, in the inventive structure, no additional amount of coupler is used in the enhancer layer. Thus, in a preferred embodiment, some of the coupler normally used in the photosensitive layer is shifted into the enhancer layer and this avoids the additional cost of using more coupler in the element.
U.S. Pat. No. 5,576,159--Sato et al describes the use of a split layer format with an enhancer layer. However, it does not describe how to solve problems associated with the use of high silver in combination with pyrazoloazole magenta couplers.